Remove Vue HAML template related code to Issue boards sidebar

There are a lot of files related to the old HAML issue templates. The new GraphQL based variant has been made default in %14.2, so hopefully we can remove it in %14.3. This blocks:

• "Entry point that needs to be removed": https://gitlab.com/gitlab-org/gitlab/-/blob/0d8100737510bce7e28dba3571b8e77bb04c7d80/app/views/shared/boards/_show.html.haml#L22